This invention relates generally to tunable capacitors, and more particularly to voltage-tuned dielectric capacitors that operate at radio frequencies.
A varactor is a voltage tunable capacitor in which the capacitance can be changed by the voltage applied thereto. This property has many applications in radio frequency (RF) circuits, such as tunable filters, phase shifters, delay lines, voltage controlled oscillators, and so on. The most commonly used varactor is a semiconductor diode varactor. However, semiconductor diode varactors suffer from low Q factors (especially at high frequencies), low power handling, and high intermodulation distortion. Common varactors used today are silicon and gallium arsenide based diodes. The performance of these varactors is defied by the capacitance ratio, Cmax/Cmin, frequency range and figure of merit, or Q factor, at the specified frequency range. The Q factors for these semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly. For example, at 10 GHz the Q factors for these varactors are usually only about 30.
Another type of tunable varactor is a dielectric varactor, whose capacitance is tuned by applying a control voltage to change a dielectric constant in a tunable dielectric material. Dielectric varactors have high Q factors, high power handling, low intermodulation distortion, wide capacitance range, and low cost.
Tunable ferroelectric materials are materials whose permittivity (more commonly called dielectric constant) can be varied by varying the strength of an electric field to which the materials are subjected. Even though these materials work in their paraelectric phase above the Curie temperature, they are conveniently called xe2x80x9cferroelectricxe2x80x9d because they exhibit spontaneous polarization at temperatures below the Curie temperature. Tunable ferroelectric materials including barium-strontium titanate (BST) or BST composites have been the subject of several patents.
Dielectric materials including barium strontium titanate are disclosed in U.S. Pat. No. 5,312,790 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Materialxe2x80x9d; U.S. Pat. No. 5,427,988 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94MgOxe2x80x9d; U.S. Pat. No. 5,486,491 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZrO2xe2x80x9d; U.S. Pat. No. 5,635,434 to Sengupta, et al. entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTO-Magnesium Based Compoundxe2x80x9d; U.S. Pat. No. 5,830,591 to Sengupta, et al. entitled xe2x80x9cMultilayered Ferroelectric Composite Waveguidesxe2x80x9d; U.S. Pat. No. 5,846,893 to Sengupta, et al. entitled xe2x80x9cThin Film Ferroelectric Composites and Method of Makingxe2x80x9d; U.S. Pat. No. 5,766,697 to Sengupta, et al. entitled xe2x80x9cMethod of Making Thin Film Compositesxe2x80x9d; U.S. Pat. No. 5,693,429 to Sengupta, et al. entitled xe2x80x9cElectronically Graded Multilayer Ferroelectric Compositesxe2x80x9d; and U.S. Pat. No. 5,635,433 to Sengupta, entitled xe2x80x9cCeramic Ferroelectric Composite Materialxe2x80x94BSTOxe2x80x94ZnOxe2x80x9d. These patents are hereby incorporated by reference. A copending, commonly assigned United States patent application titled xe2x80x9cElectronically Tunable Ceramic Materials Including Tunable Dielectric And Metal Silicate Phasesxe2x80x9d, by Sengupta, filed Jun. 15, 2000, discloses additional tunable dielectric materials and is also incorporated by reference. The materials shown in these patents, especially BSTOxe2x80x94MgO composites, show low dielectric loss and high tunability. Tunability is defined as the fractional change in the dielectric constant with applied voltage.
There are two conventional dielectric varactor structures, a planar structure and a vertical structure. The conventional planar structure varactor has a simple configuration, simple fabrication processing, and low capacitance. In planar structure varactors, a tunable film is directly deposited on a whole surface of a substrate, followed by the deposition of metal film. A metal etching process is used to obtain specific metal patterns. High processing temperatures may be necessary to achieve a high-quality tunable film. The quality of the tunable film is the main factor in determining performance of the dielectric varactor. The processing temperature of the tunable film is typically above 650xc2x0 C. for thin film, and above 1000xc2x0 C. for thick film. Since there is no bottom electrode or metal film that is subject to the high processing temperature, the conventional planar structure varactor may be more easily fabricated than the vertical structure varactor.
Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film ferroelectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film ferroelectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric material in combination with a superconducting material is disclosed in U.S. Pat. No. 5,721,194.
In some radio frequency applications, varactors having a relatively low capacitance are required. By adjusting gap size and gap width in a planar varactor, low capacitance (xcx9c1 pF) is very easily obtained. However, the bias voltage required for planar structure varactors with such relatively low capacitance can become excessive.
Using the vertical structure, wherein a layer of tunable dielectric material is positioned between two flat electrodes, can significantly reduce the required bias voltage. The bias voltage in a planar varactor is much higher than that for the vertical structure varactor, because the gap in the planar structure is usually much bigger than thickness of the tunable film in the vertical structure, especially in the case of thin film varactors.
However in a vertical structure varactor, it is difficult to achieve low capacitance, such as less than 1 pF, because of the small thickness and high dielectric constant of the tunable film. Reducing electrode area is a way to make low capacitance varactors, which are desired for many tunable devices. However, the area of the top electrode, which determines capacitance, is limited by etching processing, and the connection line (wire).
There is a need for relatively low capacitance varactors that can operate at temperatures above those necessary for superconduction and at bias voltages less than those required for existing planar varactor structures, while maintaining high tunability and high Q factors.
Varactors constructed in accordance with this invention include a substrate, a first conductor positioned on a surface of the substrate, a second conductor positioned on the surface of the substrate forming a gap between the first and second conductors, a tunable dielectric material positioned on the surface of the substrate and within the gap, the tunable dielectric material having a top surface, with at least a portion of said top surface being positioned above the gap opposite the surface of the substrate, and a first portion of the second conductor extending along at least a portion of the top surface of the tunable dielectric material. The second conductor can overlap or not overlap a portion of the first conductor.